Pluchea carolinensis
(sourbush)

Preferred Scientific Name

• Pluchea carolinensis (Jacq.) G. Don

Preferred Common Name

• sourbush

Other Scientific Names

• Conyza carolinensis Jacq.
• Pluchea odorata auct. non (L.) Cass.
• Pluchea symphytifolia auct. non Gillis

International Common Names

• English: cattletongue; coughbush; cure-for-all; curry plant; shrubby fleabane; stinking fleabane; sweet scent; wild tobacco
• Spanish: tabat-diable

Local Common Names

• Kiribati: te kaikare; te karei

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Asterales
•                         Family: Asteraceae
•                             Genus: Pluchea
•                                 Species: Pluchea carolinensis

Pluchea, named after French naturalist Noel A. Pluche (1688-1761), is a pantropical genus of aromatic shrubs or herbs in the family Asteraceae (Wagner et al., 1999). The genus, with between forty to eighty species (Eldenäs et al., 1999), is composed of several entities which have been given generic status and is regarded as heterogeneous and not monophyletic (Kadereit and Jeffrey, 2010).

There has been some confusion surrounding the nomenclature of P. carolinensis, including misapplication of the binomial P. symphytifolia (Gillis, 1977; Khan and Jarvis, 1989). This name is actually a synonym for Neurolaena lobate, another medicinal plant with an overlapping range that is used for many of the same purposes as P. carolinensis (Morton, 1981; Hodges and Bennett, 2006). The name P. odorata has also been misapplied to P. carolinensis in Texas, California and Hawaii (Wagner et al., 1999; Hodges and Bennett, 2006).

The species name carolinensis means “of Carolina” as it was erroneously believed to have originated from this region when the species was first described in 1789 (Hammer, 2004). The common name “sourbush” and others associated with smell refer to the aromatic foliage and the name “cure-for-all” derives from the many medicinal and herbal uses associated with the plant (Francis, 2004).

In the Hawaiian Islands, P. carolinensis overlaps in range and distribution with P. indica, an invasive shrub native to southern Asia. Where the two species co-occur, they are able to form spontaneous hybrids classified as P. x fosbergii (Wagner et al., 1999).

P. carolinensis is an erect, many branched, aromatic, tap rooted shrub or subshrub, 10-40 dm tall. Stems are glandular tomentose or matted villous with viscid, vitreous hairs, sometimes merely glandular.

Leaves are petiolate, dull grayish green on upper surface and pale on the lower surface. Leaf shape is elliptic to narrowly ovate or oblong-obovate, 5-20 cm long, 2-8 cm wide. The lower leaf surface is usually densely tomentose. Margins are entire or with small, callous teeth. Petioles are 1-2.5 cm long.

Flower heads are in large, flat topped inflorescences. The involucre is hemispherical, 4.5-6 mm high. The inner bracts are pubescent on the outer face and margins are ciliate. Disk corollas are whitish to pinkish lavender. The pappus is dull brownish white, consisting of a single series of 10-12 capillary bristles. Pappi are tardily falling. Achenes are dark brown, ca. 0.8 mm long, scarcely grooved and sparsely appressed whitish pubescent (Wagner et al., 1999; Flora of North America Editorial Committee, 2015).

P. carolinensis was first collected on the island of Oahu, Hawaii, in 1931 (Wagner et al., 1999). Although the exact area of origin and reason for introduction are unknown, the specimen label from the first collection describes the site as “on recent coral fill” on Honolulu (Bishop Museum, 2015). This suggests that it could have been a contaminant in construction material and likely arrived inadvertently through transoceanic shipping activities. Numerous collections in the early 1930s document its rapid dispersal across the island of Oahu and in 1933 it was collected near Hilo on Hawaii Island, possibly also as a contaminant on interisland shipping materials (Bishop Museum, 2015). Subsequent collections were made on the islands of Kauai in 1947, Lanai in 1963, Maui in 1970, Molokai in 1972 and Kahoolawe in 1980 (Bishop Museum, 2015).

The advent of World War II and the movement of large amounts of supplies, construction materials and equipment associated with military activities and installations, likely facilitated the spread of P. carolinensis throughout islands of the Pacific, with the Hawaiian Islands as the most likely seed source. This species began to be noted and collected after the end of the war on the island of Guam in 1946, Eniwetok Atoll, Marshall Islands in 1946, Majuro Atoll, Marshall Islands in 1950, Palau in 1950, Kwajalein Atoll, Marshall Islands in 1950, Tinian, Northern Mariana Islands in 1952 and several of the Northwestern Hawaiian Islands administered by the US government (Bishop Museum, 2015). The correlation between the presence of P. carolinensis and military airfields constructed during World War II has been noted as being the most likely cause of its widespread dissemination throughout its introduced Pacific range (Space and Imada, 2004; PIER, 2015).

P. carolinensis was first collected in Taiwan in 1987 and has rapidly expanded its range across the island, although the area of origin is unknown (Peng et al., 1998). It was also first noted and suspected of being accidentally introduced on the island of Gran Canaria, Canary Islands, in 2011 (Verloove, 2013). It has flowered and set seed in subsequent years and is predicted to become established and fully naturalized in the near future (Verloove, 2013).

Soil drainage

• free
• impeded
• seasonally waterlogged

Soil reaction

• acid
• alkaline
• neutral

Soil texture

• heavy
• light
• medium

Special soil tolerances

• saline
• shallow

Natural Dispersal

P. carolinensis produces prolific numbers of wind dispersed achenes (Francis, 2004). The bristly pappus primarily aids in wind dispersal, but may also provide temporary buoyancy that could facilitate secondary dispersal by water, especially in the riparian habitats, wetlands and mangrove communities where P. carolinensis commonly grows (Francis, 2004; Erickson and Puttock, 2006).

Vector Transmission

There is no evidence that the achenes of P. carolinensis are dispersed by birds, mammals or other biotic vectors, but it may be possible that the bristly pappus could aid in adherence to fur or mud on animals.

Accidental Introduction

The movement of supplies and equipment associated with military activities is suspected of being responsible for the dispersal of the seeds of P. carolinensis over long distances throughout the Pacific (Space and Imada, 2004; PIER, 2015).

Intentional Introduction

Within its native range, P. carolinensis has a history and tradition of herbal and medicinal uses (Francis, 2004; Hodges and Bennett, 2006). Such practices could result in its introduction into new areas for similar purposes. Despite the medicinal uses, however, there was no evidence found that it is widely grown or sold commercially outside of areas within its native range. 

Impact on Habitats

As an early successional species generally intolerant of shade and competition, P. carolinensis is most likely to occur in disturbed habitats, which are also negatively impacted by several other fast growing weedy plants. In these sites it may be difficult to discriminate the effects of one taxon from another. P.carolinensis, with its ability to rapidly colonize such sites and form dense monocultures, has been implicated in overall habitat degradation and displacement through competition with native species (Smith, 1985; Motooka et al., 2003). After invading burned sites of the Hawaiian Islands, it is however usually outcompeted and replaced by later successional plants (Smith and Tunison, 1992).

P.carolinensis is also considered to be a primary threat to anchialine pools throughout the coastal areas of the Hawaiian Islands (Pratt and Abbott, 1996). In particular, it produces abundant leaf litter that can contribute to increased eutrophication and early pool senescence (Chai et al., 1989).

Impact on Biodiversity

P. carolinensis primarily invades areas of natural and anthropogenic disturbance, along with several other early successional weedy taxa. Therefore, its impacts on biodiversity in these sites may be minimal. Nevertheless, with its ability to form dense monocultures, it can monopolize resources, displace native flora and interfere with native plant succession in invaded sites. As a result, it has been identified as one of the primary weed threats to 12 species or subspecies of proposed or endangered plant taxa in the Hawaiian Islands. These include Abutilon eremitopetalum, Bidens hillebrandiana subsp. hillebrandiana [B. hillebrandiana], B. micrantha subsp. ctenophylla,Cyanea undulata,Cyrtandra munroi, Dubautia pauciflorula,Euphorbia celastroides var. kaenana,Kadua st.johnii, Labordia triflora,Melanthera micrantha subsp. exigua, M. micrantha subsp. micrantha and Pittosporum halophilum (Wood and Kiehn, 2011; US Fish and Wildlife Service 2000; 2003; 2007; 2008; 2012).

• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Highly adaptable to different environments
• Is a habitat generalist
• Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
• Pioneering in disturbed areas
• Gregarious

• Ecosystem change/ habitat alteration
• Modification of successional patterns
• Monoculture formation
• Negatively impacts agriculture
• Reduced native biodiversity
• Threat to/ loss of endangered species
• Threat to/ loss of native species

• Competition - monopolizing resources
• Competition - shading
• Pest and disease transmission
• Hybridization
• Rapid growth

Economic Value

Within its native range, P. carolinensis is used medicinally and has economic value to the botanicas and herbal shops that sell it in the form of dried powders, cut plants, live plants and oil extracts (Francis, 2004; Hodges and Bennett, 2006).

Environmental Services

P. carolinensis can rapidly colonize and stabilize disturbed areas and may serve as a nurse crop for later successional species within its native range (Francis, 2004). This species is also regularly visited by honeybees and may therefore be an important nectar and pollen source for these and other insects (Cate, 2007).